Technical Field
The present invention relates to a projection device to magnify and project an image displayed at an image display element on a projection surface such as a screen, and a projection system including the projection device.
Background Art
A projection device illuminates a display screen of an image display element referred to as a light valve such as a Digital Micromirror Device (abbreviated as DMD) and a Liquid Chrystal Panel, and projects a magnified image displayed at the image display element on a screen forming the projection surface by a projection optical system.
Especially in recent years, there is a growing demand for a front projection projector having an ultra-short projection distance by which a large-size image can be displayed with a short projection distance. As a unit having a compact size and achieving such an ultra-short projection distance, there are technologies proposed in, for example, JP-2007-079524-A, JP-2009-251458-A, JP-2011-242606-A, and JP-2009-216883-A, in which a dioptric system and a curved mirror are combined.
Recently, higher luminance is strongly demanded even in such an ultra-short throw projector. It is thus also desired to sufficiently consider temperature characteristics of the optical system, due to heat from a lamp or a light source, and heat generated through absorbing beams.
Particularly, since a projection angle is large in the ultra-short throw projector, a focus depth has only a few centimeters or so in a periphery, particularly at a point most distant from an optical axis which is the axis shared by the dioptric system. That is, the focus depth is extremely narrowed in the ultra-short throw projector, compared to the front projection projectors not having an ultra-short projection distance. Accordingly, the ultra-short projector tends to suffer from degradation in resolution as a focus position is largely deviated in the periphery of a screen, due to an imaging surface curvature caused by the above-mentioned temperature increase.
However, nothing is disclosed about correcting the imaging surface curvature caused by temperature increase in the above-listed JP-2007-079524-A, JP-2009-251458-A, JP-2011-242606-A, and JP-2009-216883-A, and the technologies are insufficient in the case of considering the specifications of the recent projectors.